1. Field of the Invention
Embodiments of the present invention relate generally to integrated circuit bus interfaces and more specifically to a bus protocol for transferring pixel data between chips.
2. Description of the Related Art
A graphics processing unit (GPU) typically includes at least one real-time video output port. An analog video output port incorporates a digital-to-analog converter (DAC) for generating analog video signals that are transmitted through output pins on the GPU to a display device, such as an LCD monitor. Incorporating at least one analog video output port is currently a requirement on high-volume GPU devices. Another type of real-time video output port incorporates a high-speed serial output resource for transmitting real-time video signals through output pins on the GPU to a display device. While both types of real-time video output ports are generally susceptible to on-chip noise generated by on-chip circuitry switching, the analog video output port is particularly sensitive. On-chip noise that couples to the circuitry associated with the analog real-time video output port can significantly degrade the quality of both the resulting analog video output signal and the final video image. Because noise naturally couples from a noise source, such as actively switching on-chip logic, to a noise victim, such as an on-chip analog circuit, substantial engineering effort is typically required to reduce noise in the analog video output signal.
As more logic gates are integrated into successive generations of GPU devices, the on-chip noise generated by switching logic will likely increase, thereby increasing the potential for switching noise to be coupled into the circuitry associated with an analog real-time video output port. The noise may be coupled through more than one mechanism. For example, substrate currents, electromagnetic coupling and inductive coupling may each inject significant noise into the analog real-time video output port, making mitigation strategies progressively less effective as more logic is integrated into future devices and more overall noise is generated in these devices.
One solution is to physically separate the switching logic and the sensitive analog video circuitry. However, this approach is costly in terms of die area, especially in systems with two independent analog video output ports that are used to support two different monitors. Another solution is to aggressively shield the analog video circuitry from the switching logic with on-chip metal barriers. However, these shields are generally not effective against inductively coupled noise, a significant contribution to overall coupled noise within an integrated circuit.
As the foregoing illustrates, what is needed in the art is a system that decouples noise sources from analog video output circuitry in a GPU, while minimizing overall design effort.